A problem that frequently arises in the packaging industry is that of producing a packaging material consisting of more than one layer, for example, coating cardboard or paper with plastics film to provide mechanical protection, or laminating two or more sheets together in order to prevent, for example, permeability to gas and/or liquid.
In many cases welding techniques or solvent-containing adhesives have previously been used for those purposes. It is not necessary to go into detail here about the disadvantages of surface welding and equally, quite apart from the economic disadvantages, there are evident problems in the use of solvent-containing adhesives as regards environmental pollution, work place sanitation and the fire risk.
To avoid those disadvantages, an adhesive that is said to solve these problems was proposed in Japanese Published Application JP-OS No. 77/42532 (according to a review in Chemical Abstracts 87 [1977] 40 402 k). The adhesive used in that case consists of an aqueous dispersion of an ethylene/vinylacetate copolymer (PEVAC), into which 5% to 30% by weight of epoxy resins, based on the PEVAC content, have been incorporated, and to which an amine hardener has been added.
The adhesive was produced, for example, by dissolving a mixture of an aqueous solution of protective colloids and sodium acetate with epoxy resin in vinyl acetate, adding ethylene, polymerizing by means of a water-soluble redox catalyst system, and then adding the polyamine.
The use of such adhesives gives quite satisfactory results as regards resistance to peeling provided that the laminate is not subjected to thermal stress and/or stress as a result of moisture.